I only have to finish Dead Orbit to have all factions at least level 3, should have it done tonight hopefully. The benefits are the ability to purchase stat specific legendary armor (lvl 33 of course). Also its a good way to get decent weapons rather than relying on RNG. Each faction has their stat focus. Datto and a few other's suggest at least leveling up FWC because of the buffs to intellect and discipline. Its totally up to you. Only downfall though is if you don't play PvP you are wasting your time leveling up factions as they require crucible marks and commendations.